Flowers Again

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Let's now, in the third of our warm-up forays into natural selection, move on to flowers and pollinators and see something of the power of natural selection to drive evolution. Pollination biology furnishes us with some pretty amazing facts, and the high point of wondrousness is reached in the orchids. No wonder Darwin was so keen on them; no wonder he wrote the book I have already mentioned, The Various Contrivances by which Orchids are Fertilised by Insects. Some orchids, such as the 'magic bullet' Madagascar ones we met earlier, give nectar, but others have found a way to bypass the costs of feeding pollinators, by tricking them instead. There are orchids that resemble female bees (or wasps or flies) well enough to fool males into attempting to copulate with them. To the extent that such mimics resemble females of one particular insect species, to that extent will males of those species serve as magic bullets, going from flower to flower of just the one orchid species. Even if the orchid resembles 'any old bee' rather than one species of bee, the bees that it fools will still be 'fairly magic' bullets. If you or I were to look closely at a fly orchid or a bee orchid (see colour page 5), we would be able to tell that it was not a real insect; but we would be fooled at a casual glance out of the corner of our eye. And even looking at it head-on, I would say the bee orchid in the picture (h) is pretty clearly more of a bumble-bee orchid than a honey-bee orchid. Insects have compound eyes, which are not so acute as our camera eyes, and the shapes and colours of insect-mimicking orchids, reinforced by seductive scents that mimic those of female insects, are more than capable of tricking males. By the way, it is quite probable that the mimicry is enhanced when seen in the ultraviolet range, from which we are cut off.

The so-called spider orchid, Brassia (colour page 5 (k)), achieves pollination by a different kind of deception. The females of various species of solitary wasp ('solitary' because they don't live socially in large nests like the familiar autumn pests, called yellowjackets by Americans) capture spiders, sting them to paralyse them, and lay their eggs on them as a living food supply for their larvae. Spider orchids resemble spiders sufficiently to fool female wasps into attempting to sting them. In the process they pick up pollinia - masses of pollen grains produced by the orchids. When they move on to try to sting another spider orchid, the pollinia are transferred. By the way, I can't resist adding the exactly backwards case of the spider Epicadus heterogaster, which mimics an orchid. Insects come to the 'flower' in search of nectar, and are promptly eaten by it.

Some of the most astonishing orchids that practise this seduction trick are to be found in Western Australia. Various species in the genus Drakaea are known as hammer orchids. Each species has a special relationship with a particular species of wasp of the type called thynnids. Part of the flower bears a crude resemblance to an insect, duping the male thynnid wasp into attempting to mate with it. So far in my description, Drakaea is not dramatically different from other insect-mimicking orchids. But Drakaea has a remarkable extra trick up its sleeve: the fake 'wasp' is borne on the end of a hinged 'arm', with a flexible 'elbow'. You can clearly see the hinge in the picture (colour page 5 (g)). The fluttering movement of the wasp gripping the dummy wasp causes the 'elbow' to bend, and the wasp is dashed repeatedly back and forth like a hammer against the other side of the flower - let's call it the anvil -where it keeps its sexual parts. The pollinia are dislodged and stick to the wasp, who eventually extricates himself and flies off, sadder but apparently no wiser: he goes on to repeat the performance on another hammer orchid, where he and the pollinia he bears are duly dashed against the anvil, so that his cargo finds its destined refuge on the female organs of the flower. I showed a film of this astounding performance in one of my Royal Institution Christmas Lectures for Children, and it can be seen in the recording of the lecture called 'The Ultraviolet Garden'.

In the same lecture I discussed the 'bucket orchids' of South America, which achieve pollination in an equally remarkable but rather different way. They too have specialized pollinators, not wasps but small bees, of the group called Euglossine. Again, these orchids provide no nectar. But the orchids don't fool the bees into mating with them either. Instead, they provide a vital piece of assistance for male bees, without which the bees would be unable to attract real females.

These little bees, which live only in South America, have a strange habit. They go to elaborate lengths to collect fragrant, or anyway smelly, substances, which they store in special containers attached to their enlarged hind legs. In different species these smelly substances can come from flowers, from dead wood, or even from faeces. It seems that they use the gathered perfumes to attract, or otherwise court, females. Many insects use particular scents to appeal to the opposite sex, and most of them manufacture the perfumes in special glands. Female silk moths, for example, attract males from an astonishingly long distance by releasing a unique scent, which they manufacture and which males detect - in minute traces from literally miles away - with their antennae. In the case of Euglossine bees, it is the males that use scent. And, unlike the female moths, they don't synthesize their own perfume but use the smelly ingredients that they have collected, not as pure substances but as carefully concocted blends which they put together like expert perfumiers. Each species mixes a characteristic cocktail of substances gathered from various sources. And there are some species of Euglossine bee that positively need, for manufacturing their characteristic species scent, substances that are supplied only by flowers of particular species of the orchid genus Coryanthes - bucket orchids. The common name of Euglossine bees is 'orchid bees'.

What an intricate picture of mutual dependence. The orchids need the Euglossine bees, for the usual 'magic bullet' reasons. And the bees need the orchids, for the rather weirder reason that they can't attract female bees without substances that are either impossible or at least too hard to find except through the good offices of bucket orchids. But the way in which pollination is achieved is even weirder still, and it superficially makes the bee look more like a victim than a cooperating partner.

A male Euglossine bee is attracted to the orchids by the smell of the substances that he needs in order to manufacture his sexual perfumes. He alights on the rim of the bucket and starts to scrape the waxy perfume into the special scent pockets in his legs. But the rim of the bucket is slippery underfoot - and there's a reason for this. The bee falls into the bucket, which is filled with liquid, in which he swims. He cannot climb up the slippery sides of the bucket. There is only one escape route, and this is a special bee-sized hole in the side of the bucket (not visible in the picture that appears on colour page 4). He is guided by 'stepping stones' to the hole and starts to crawl through it. It's a tight fit, and it becomes even tighter as the 'jaws' (these you can see in the picture: they look like the chuck of a lathe or electric drill) contract and trap him. While he is held in their grip, they glue two pollinia to his back. The glue takes a while to set, after which the jaws again relax and release the bee, who flies off, complete with pollinia on his back. Still in search of the precious ingredients for his perfumery, the bee lands on another bucket orchid and the process repeats itself. This time, however, as the bee struggles through the hole in the bucket, the pollinia are scraped off, and they fertilize the stigma of this second orchid.

The intimate relationship between flowers and their pollinators is a lovely example of what is called co-evolution - evolution together. Co-evolution often occurs between organisms that have something to gain from each other, partnerships in which each side contributes something to the other, and both gain from the cooperation. Another beautiful example is the set of relationships that have grown up around coral reefs, independently in many different parts of the world, between cleaner fish and larger fish. The cleaners belong to several different species, and some are not even fish at all but shrimps - a nice case of convergent evolution. Cleaning, among coral-reef fish, is a well-established way of life, like hunting or grazing or anteating among mammals. Cleaners make their living by picking parasites off the bodies of their larger 'clients'. That the clients benefit has been elegantly demonstrated by removing all the cleaners from an experimental area of reef, whereupon the health of lots of species of fish declines. I have discussed the cleaning habit elsewhere, so will say no more here.

Co-evolution also occurs between species that don't benefit from each other's presence, like predators and prey, or parasites and hosts. These kinds of co-evolution are sometimes called 'arms races' and I postpone discussing them to Chapter 12.

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